Abstract B177: The discovery and preclinical characterization of AMG 193, a first-in-class MTA-cooperative PRMT5 inhibitor with broad activity against MTAP-null cancers

Abstract Homozygous deletion of the methylthioadenosine phosphorylase (MTAP) gene occurs in approximately 15% of all cancers due to its proximity to the commonly deleted tumor suppressor gene CDKN2A. Elevated methylthioadenosine (MTA) levels, driven by loss of MTAP, compete with the methyl donor S-adenosylmethionine (SAM) for binding to protein arginine N-methyltransferase 5 (PRMT5), placing the methyltransferase in a hypomorphic state, and vulnerable to further PRMT5 inhibition. The screening of a DNA-encoded library, designed to identify small molecules that preferentially bind to PRMT5 in the presence of MTA, resulted in the identification of AM-9934 as a promising hit. Using structure-based drug design, potency and cooperativity were enhanced to deliver AM-9747, an MTA-cooperative PRMT5 inhibitor suitable for in vivo proof-of-concept studies. Further optimization led to AMG 193, an orally bioavailable and MTA-cooperative PRMT5 inhibitor with potent biochemical and cellular activity, and an improved pharmacokinetic profile. Insights into the binding mode of AMG 193 were elucidated by the crystal structure of AMG 193 bound to the MTA-PRMT5 complex. AMG 193 activity is enriched in MTAP-null cells in vitro, as illustrated in the HCT116 isogenic pair; AMG 193 is ~40X selective for MTAP-null cells (IC50 = 0.1 mM) over MTAP WT cells (IC50 > 4 mM) in viability assays and >100X selective in an assay evaluating symmetric dimethylarginine (SDMA) levels. AMG 193 was profiled in a panel of >850 cancer cell lines, and response demonstrated preferential sensitivity in MTAP-null cells compared to WT across multiple cancer lineages, including pancreatic, lung, biliary tract, and lymphoma. Response strongly correlated with both MTAP and CDKN2A copy number loss, and cell lines that were dependent on PRMT5, as shown by RNAi and CRISPR knockdown, were most sensitive to AMG 193 treatment. In vitro mechanism-of-action studies demonstrated that PRMT5 inhibition induced DNA damage, cell cycle arrest in G2/M, and an increase in alternative mRNA splicing in MTAP-null cells. In mice oral, once-daily administration of AMG 193 selectively inhibited SDMA in tumor versus normal tissue and was well tolerated with no hematologic perturbations. Importantly, AMG 193 demonstrated robust anti-tumor activity across a broad range of MTAP-null CDX and PDX tumor models derived from NSCLC, pancreatic, and esophageal cancers. In vitro synergy was observed with the combination of AMG 193 and several chemotherapeutic agents, including carboplatin and paclitaxel, or the KRASG12C inhibitor, sotorasib. In vivo, these combinations led to enhanced anti-tumor activity relative to single agent in MTAP-null NSCLC tumor models. In February 2022, AMG 193 became the first MTA-cooperative PRMT5 inhibitor to enter clinical development and is currently being evaluated in subjects with advanced MTAP-null solid tumors in the ongoing FIH study (NCT05094336). Citation Format: Brian Belmontes, Katherine Slemmons, Siyuan Liu, Antonia Policheni, Jodi Moriguchi, Hong Tan, Fang Xie, Famke Aeffner, Matthew G Rees, Melissa M Ronan, Jennifer A Roth, Mikkel Vestergaard, Sanne Cowland, Jan Andersson, Ian Sarvary, Patricia Lopez, Nuria Tamayo, Liping H Pettus, Susmith Mukund, Jennifer R Allen, Sanne Glad, Matthew P Bourbeau. The discovery and preclinical characterization of AMG 193, a first-in-class MTA-cooperative PRMT5 inhibitor with broad activity against MTAP-null cancers [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr B177.